When birth control isn’t covered under a female student’s insurance policy, she is slightly less likely to have sex, but is – in some cases -- significantly more likely to use less reliable contraception methods.

Those are the findings of a University of Michigan study published last year, first reported Thursday by Slate[1] in response to conservative commentator Rush Limbaugh’s suggestion that women should either pay for birth control themselves or just have less sex.

Limbaugh was referring to Georgetown University law student Sandra Fluke[2], who has unwittingly found herself in the center of a firestorm[3] after speaking in favor of President Obama’s birth control mandate, which would require contraception to be covered in all insurance plans. Fluke, of course, has pointed out that many women use the pill for health reasons other than preventing pregnancy. Nearly 40 percent of female college students are on the Pill, according to the American College Health Association, and of all those who use some kind of birth control, three in five choose the pill.

The study[4], released in May 2011 by Emily Gray Collins and Brad Hershbein of the University of Michigan’s Population Studies Center[5], aimed to explore the previously unaddressed question of how women’s sexual behavior changes when the price of oral contraception goes up. The fact that women are less likely to use the pill when it is more expensive had already established in previous literature.

Collins and Hershbein used data from four-year colleges reported in the National College Health Assessment to look at how behavior shifted after Jan. 1, 2007, when the Deficit Reduction Act of 2005 went into effect. That legislation made college health clinics ineligible for the previous pricing deal they’d had with pharmaceutical companies, in which campuses could buy medications to resell to students at deeply discounted prices. As a result, the price of birth control on campuses rose from $5-10 for a month’s supply to $30-50, the study says.

(While Planned Parenthood was not affected by the price increase, the study noted that the organization “in particular, is often used by college women,” which is notable considering that Planned Parenthood has become the target of politicians who want to defund it, and already have, in some states[6].)

The study found that on average, women reduced their use of the pill by 1 to 1.8 percentage points – that’s 2 to 4 percent of women – after costs went up. But specifically for uninsured women – which, should religious institutions stop covering contraception[7], would include women on student health plans at those colleges – the decline of pill use was at least double that, as 3.5 to 8 percent of women stopped using it, a shift of 2.0 to 3.6 percentage points. Only 1.5 to 2 percent of insured women stopped using the pill, a decrease of 0.9 to 1.2 percentage points. That is because nearly all insurance policies cover birth control pills, so insured women would have more alternative places to get contraception than would uninsured women, who would be limited to a smaller set of clinics that provide low-cost pills.

“Assuming women without insurance faced a larger price increase, this is consistent with our model that the larger the price change, the more likely a woman is to change her optimal birth control choice,” the study says.

Uninsured women who didn’t have sex frequently turned at the highest rates to emergency contraception like Plan B, or the morning after-pill, which does not induce abortion but rather prevents fertilization of the egg or makes it more difficult. (Plan B has been particularly contentious of late[8] on college campuses.) But uninsured women who had sex more often and stopped using the pill opted for “non-prescription forms of birth control” – in other words, riskier methods like rhythm or withdrawal.

“This is consistent with the idea that women who are having sex, or having more sex, are deriving more benefit from the pill and would thus be willing to pay a higher price to continue using it,” the report says. For those who are having sex less often, “purchasing EC on those occasions might be less expensive than purchasing continuous birth control every month.”

The researchers also identified a negative relationship between credit card debt and birth control choice: the higher a woman’s debt, the less likely she was to stick with her optimal form of contraception. They explained their thinking: “Credit card debt can be thought of as a proxy for financial constraints; women with large credit card balances are more likely to be both lower income and more credit constrained and price sensitive than their peers without balances.”

Roughly half of women who were uninsured and or had high credit card debt and who stopped using the pill switched to non-prescription methods.

Women with credit card balances of less than $2,000 reduced their use of the pill by 1.5 to 2.3 percentage points, compared to a reduction of 3.5 to 4.6 percentage points – a nearly 10 percent decrease in the fraction of women using the pill – for women with debt over $2,000. The likelihood that a woman will switch methods increases with the amount of debt she has.

“These results point to a pattern of women at risk of financial disadvantage, who were the women most likely to benefit from the subsidies at campus health centers, having strong and statistically significant reductions in their usage of the pill in response to the price increase, while more advantaged women were more likely to absorb the price increase,” the study says. “For women without insurance and women with large amounts of debt, for whom the price increase is more likely to bind, we estimate the change in demand for the pill to between 5 and 10 percent, in response to a price increase of at least 300 percent.”

In other findings, the researchers did not identify statistically significant changes in the number of sexually transmitted diseases or accidental pregnancies. But they also found “small but significant” decreases in frequency of intercourse and number of sex partners.

James Trussell, faculty associate of Princeton University’s Office of Population Research and creator of its educational emergency contraception website[9], said the study is “really important” and points to a clear conclusion.

“When contraception is covered as the Affordable Care Act is implemented,” Trussell said in an e-mail, referring to Obama’s health care overhaul that includes the contraception mandate and allows students to stay on their parents’ plans until age 26, “it is highly likely that women in college will switch to more effective methods.”